1. Field of the Invention
This invention relates to laminates comprising a thermoplastic core layer and metal layers (also referred to as facings) laminated on each side of the core. More particularly, this invention relates to lightweight metal-thermoplastic-metal laminates having a polyamide or polyester core and metal facings which are aluminum alloys.
2. Description of the Prior Art
Metal-thermoplastic-metal laminates and processes for their preparation have been disclosed in the prior art. See, for example, U.S. Pat. No. 3,382,136 issued May 7, 1968 to T. E. Bugel et al.
Mor recently, in U.S. patent application Ser. No. 99,918, filed Dec. 14, 1979, which was abandoned in favor of continuation application Ser. No. 230,934 filed Feb. 2, 1981, by R. M. Hedrick et al, there are disclosed metal-thermoplastic-metal laminates having a combination of properties which make the laminates particularly suitable for use as construction panels. These laminates employ as the thermoplastic core material a partly crystalline polyamide or polyester having a crystalline melting point of at least about 130.degree. C. The metal layers laminated on each side of the thermoplastic core layer have a melting point greater than the thermoplastic core layer and a minimum thickness of about 0.00127 cm. Metals such as carbon steel, stainless steel and aluminum are disclosed. There is also disclosed a correlation between the metal layer-thickness to the thermoplastic core layer thickness. The laminates described in Hedrick et al. are lightweight and exhibit a combination of inelastic ratio, coefficient of linear thermal expansion and heat distortion temperature which make them particularly suitable for use as construction panels.
Although the teachings in Hedrick et al represent a significant advance in the art related to metal-thermoplastic metal laminates, further improvements in such laminates would be of interest. For example, laminates which combine good bending strength and good formability would be of interest to those skilled in the art. Formability of a laminate is an important property in applications where the laminate is to be formed into a shaped structure subsequent to preparation of the laminate. At the same time, it is generally desirable for a laminate to have high bending strength so that parts made therefrom maintain their shape under applied stress. Generally, improving formability of the laminate is at the expense of bending strength.
It is an object of the present invention to provide laminates with a combination of good formability and bending strength. It is a further object of this invention to provide laminates which exhibit improved dent resistance. These and other objects of the present invention will become apparent from the disclosure which follows.